Catalytic cracked gasoline is an important gasoline mixture source. Namely, catalytic cracked gasoline has a high olefin content of from 20 to 40% and therefore has a high octane number, and constitutes up to 50% of the raw material of a gasoline. The catalytic cracked gasoline is produced by catalytic cracking a vacuum gas oil and an atmospheric residual oil using a fluidized catalytic cracking apparatus. In this production process, sulfur contained in these heavy oils is also subjected to various cracking reactions. Thus, the catalytic cracked gasoline is characterized as including sulfur compounds.
In order to suppress the sulfur content of catalytic cracked gasoline to a low level, generally hydrodesulfurized vacuum gas oil and atmospheric residual oil are used as stock oils (feedstocks) for catalytic cracking. However, the hydrodesulfurization apparatus of these heavy oils is used at high temperature and under high pressure. Therefore, in order to respond to continuously imposed severe regulations for protecting the environment, the provision of new or additional facilities as well as the increase in capacity of existing apparatuses disadvantageously requires a large investment.
Sulfur compounds contained in catalytic cracked gasoline can be hydrodesulfurized by means of an apparatus which can be operated at a relatively low temperature and under a relatively low pressure. Therefore, if catalytic cracked gasoline can be directly hydrodesulfurized, the investment in plant and equipment can be relatively inexpensive and also the operating cost is advantageously lower than that of the hydrodesulfurization of heavy oil.
When catalytic cracked gasoline is hydrodesulfurized according to a conventional technique, e.g., using a naphtha hydrodesulfurization apparatus, olefins are disadvantageously hydrogenated to decrease the octane number. In order to prevent this decrease in octane number, various methods have been proposed, e.g., a technique in which a stock oil is separated into a light fraction and a heavy fraction by distillation and the resulting respective fractions are hydrodesulfurized under different conditions (U.S. Pat. No. 4,990,242); a method using a catalyst modified with an alkali metal (JP-A-8-277395); a method for preventing a decrease in the octane number by using an acidic catalyst comprising synthetic zeolite (U.S. Pat. No. 5,352,354); and a method using a catalyst subjected to a regular pre-treatment (U.S. Pat. No. 4,149,965). However, these techniques have various problems, and their performance is not sufficient to prevent a decrease in the octane number.
It is therefore an object of the present invention to solve the above-described problems of the prior art, and to provide a method for effectively conducting hydrodesulfurizion while preventing the hydrogenation of olefins, as well as to provide a catalytic cracked gasoline processed by the hydrodesulfurization method.